what are optical fibres

It is a glass or plastic core surrounded by a cladding made up of a similar material but with a lower refractive index. Its diameter is much small compared to its length.

 When light enters this optical fibre, it strikes the interface in denser medium at an angle, that is greater than the so called ‘critical angle’ and is totally reflected back in the same denser medium .This happens each time the light ray strikes the wall and it travels inside the fibre. Only when it reaches the other end, it is allowed to exit the fibre.